A typical papermaking process includes the steps of: 1) pulping wood or some other source of papermaking fibers; 2) producing a paper mat from the pulp, the paper mat being an aqueous slurry of cellulosic fiber which may also contain additives such as inorganic mineral fillers or pigments; 3) depositing this slurry on a moving papermaking wire or fabric; 4) forming a sheet from the solid components of the slurry by draining the water; 5) pressing and drying the sheet to further remove water, and 6) potentially rewetting the dry sheet by passing it through a size press and further drying it to form a paper product.
When conducting a papermaking process, a number of concerns need to be taken into account to assure the quality of the final paper product. For example, when draining the water from the slurry, fibers and chemical additives should be retained as much as possible instead of flowing away with the water. Similarly, the final sheet should have adequate wet strength and dry strength. The dry strength of paper generally includes, for example, internal bonds, dry tensile strength, and burst strength.
U.S. Pat. Nos. 8,465,623, 7,125,469, 7,615,135 and 7,641,776, which are incorporated herein by reference in their entirety, present some materials that could be used as dry strength agents. These agents can be added to the slurry to increase the strength properties of the final sheet. These agents should be capable of improving the paper machine drainage without interfering or hurting the effectiveness of other additives added in paper making process.
Commonly used dry strength agents include natural polymers, such as cationic starch, carboxymethyl cellulose (CMC) and guar gum, and synthetic polymers, such as polyacrylamide (cationic, anionic and amphoteric), glyoxalated polyacrylamides (GPAMs), and polyvinylamines. In the category of di-aldehyde functionalized polyacrylamide, glyoxalated polyacrylamide (GPAM), prepared from glyoxal and a polyacrylamide backbone, is the most commonly used dry strength agent.
Anionic, amphoteric, and cationic di-aldehyde functionalized polyacrylamides (mostly GPAMs) can usually be used alone (see, for example, WO 00/11046, U.S. Pat. Nos. 7,641,766, and 7,901,543, all of which are incorporated herein by reference in their entirety). However, single dry strength agents usually can't meet all necessary requirements so complex dry strength agents are being developed. For example, U.S. Patent Application Publication No. 2008/0196851, which is incorporated herein by reference, provides a method for improving the dry strength of paper in which a composition comprising at least two kinds of dry strength agents is used. The first dry strength agent corresponds to a Hoffmann degradation product deriving from a base polymer containing at least one non-ionic monomer while the second dry strength agent corresponds to a polymer with an anionic charge density greater than 0.1 meq/g. However, the product cost is high because of the usage of a Hoffmann degradation product, which is prepared by a complicated process.